1. Field of the Invention
The present disclosure relates to a developing device that develops an electrostatic latent image on a latent image bearing member with a developer including toner and a magnetic carrier. In particular, the present disclosure relates to a developing device that is used in an image forming apparatus such as a copying machine, a printer, a facsimile machine (FAX), and a multifunction peripheral having a plurality of such functions.
2. Description of the Related Art
An electrophotographic image forming apparatus includes a developing device for developing an electrostatic latent image formed on a charged photosensitive member. The developing device includes a developing sleeve, which is rotatably arranged in a position opposite the photosensitive member. The developing sleeve conveys a developer to a developing position opposite the photosensitive member, whereby the electrostatic latent image on the photosensitive member is developed.
The developer is stored in a developing container. Agitation screws agitate and circulate the developer in the developing container. Some of the developer may move in the axial direction of the developing sleeve to leak or scatter through the ends of the developing sleeve. A method for attaching elastic seal members to both ends of the developing sleeve in the developing container has conventionally been discussed to seal the ends so that the toner will not leak through the ends of the elastic seal members. For example, the elastic seal members may be pressed against the outer periphery of the developing sleeve for sealing. Pressing the elastic seal members against the outer periphery of the developing sleeve, however, causes a high load on the developing sleeve. Another problem is that the elastic seal members deteriorate and decrease in the sealability.
Some developing devices using toner or a carrier capable of magnetic attraction are configured to include magnetized magnetic seal members. The magnetic seal members are arranged on surfaces opposed at a predetermined distance to the surface of the developing sleeve, and magnetically attract and hold the developer for magnetic sealing (for example, see Japanese Patent Application Laid-Open No. 11-133750). Such a magnetic seal configuration has the advantage of reducing the rotation load of the developing sleeve since the developing sleeve and the magnetic seal members are not in contact with each other. The magnetic seal configuration also has the advantage of having long life due to the absence of abrasive deterioration.
According to the configuration of the developing device discussed in Japanese Patent Application Laid-Open No. 11-133750, the developer may leak from one of the ends of the developing sleeve after image formation on a large number of sheets. The problem is described below. As illustrated in FIG. 3, a developing device using a two-component developer is configured to circulate and convey the developer by agitation members 42 and 43 arranged in a developing chamber 41a and an agitation chamber 41b. The first agitation member 42 conveys the developer stored in the developing chamber 41a in the direction of the arrow. In the developing chamber 41a, the developer, therefore, flows more into the end portion of the developing sleeve 44 on the downstream side 41e in the conveyance direction of the developer and the pressure of the developer becomes higher than on the upstream side 41d in the conveyance direction of the developer. Such a phenomenon is found to appear as the leakage of the developer on the downstream side 41e in the conveyance direction of the developer after image formation on a large number of sheets.
To increase the sealability so that the developer conveyed by the first agitation member 42 arranged in the developing chamber 41a will not leak from the downstream side 41e in the conveyance direction of the developer, the magnetic seal members may be configured to have a higher magnetic force. However, increasing the magnetic force of the magnetic seal members can in turn cause a toner leakage on the upstream side 41d in the conveyance direction of the developer. A possible mechanism of the toner leakage is described below. In areas where the magnetic seal members strongly bind the developer, some of the developer forms an immobile layer. The boundary between the immobile layer portion and a fluid portion of the developer moved by the developing sleeve 44 tends to cause friction and fixing of the toner, whereby the toner coagulates easily. The coagulated toner is considered to leave from the carrier and eventually cause a toner leakage. The toner leakage is found to be particularly likely to occur on the upstream side 41d in the conveyance direction of the developer. The reason is that in the developing chamber 41a, the developer flows less into the end portion of the developing sleeve 44 on the upstream side 41d in the conveyance direction of the developer and thus a smaller amount of developer is replaced than on the downstream side 41e in the conveyance direction of the developer. Of the magnetic seal portions at both ends, the toner is considered to be more likely to coagulate in the magnetic seal portion on the upstream side 41d in the conveyance direction of the developer than on the downstream side 41e in the conveyance direction of the developer.
A developing device of the vertical agitation type, including a developing chamber and an agitation chamber arranged one above the other, has a similar problem. With the vertical agitation developing device, the developer convey-up side of the developer circulation path is more susceptible to the developer pressure than the convey-down side. The developer is more likely to leak from the developer convey-up side. The magnetic flux densities of the magnetic seal members at both ends of the developing sleeve may be increased to suppress the leakage of the developer, in which case the toner can coagulate to cause a toner leakage.